1. Field of the Invention
The present invention relates to a control apparatus for an automatic transmission suitably used in an automotive vehicle.
2. Description of the Related Art
In general, an automatic transmission for an automotive vehicle becomes popular in which a revolution of an engine is inputted via a torque converter, a gear shift mechanism having a plurality of couples of planetary gears performs a gear shift for the inputted revolution, and the gear shifted revolution is outputted to a drive shaft or a propeller shaft. The gear mechanism in such a kind of automatic transmission as described above executes a gear shift by transmitting a revolution of an input shaft to a specific gear or a carrier constituting the planetary gear according to a shift position and by transmitting the revolution of the specific gear or carrier appropriately to an output shaft. In addition, the gear shift mechanism, to constrain the revolution of the specific gear or carrier during the gear shift, frictional clutching (or engagement) elements such as a plurality of clutches and brakes are provided. A combination of clutching (coupling or engagement) or release of these frictional clutching elements switches a transmission route to perform a predetermined gear shift. Hydraulic multi-plate clutch mechanisms have widely been adopted as these frictional clutching elements. Each hydraulic multi-plate clutch mechanism is mainly constituted by a clutch having a plurality of frictional plates and a piston as an actuator to bring in a close contact with the clutch. This piston presses the frictional plates and moves in a closely contact direction by supplying a working oil to a working oil chamber formed between cylinders. When the working oil pressure supply to the working oil chamber is stopped, a restoring force of a return spring causes the frictional plates to be recovered to a non-operation position at which the piston does not press the frictional plates. In addition, during an operation of the piston, a, so-called, ineffective stroke is present until the piston is brought in contact with the clutch. However, in order to eliminate this ineffective stroke as quickly as possible, the working oil pressure (the hydraulic) under a high pressure is once supplied to the oil pressure chamber until the stroke of the piston is ended and, thereafter, the oil pressure (the hydraulic) under a relatively low pressure is supplied. It is noted that the oil pressure or supply time at this time is set to an appropriate oil pressure by a tuning for the engine output torque or its corresponding parameter so that an appropriate clutching (engagement) operation becomes possible.
However, it is a general practice that the piston and the cylinder to support slidably the piston are revolved together with the drive element to be engaged or a driven element. Hence, a pressure due to a centrifugal force (hereinafter, referred to as a centrifugal hydraulic) is developed in the working oil chamber. A trouble often occurs in a shift operation depending upon the piston or the revolution speed of the cylinder. That is to say, due to the development of the centrifugal hydraulic, the working oil pressure actually developed is higher than an intended working oil pressure. Thus, a shift shock occurs. As a measure against the centrifugal hydraulic, a Japanese Patent Application First Publication No. Heisei 2-292566 published on Dec. 4, 1990 exemplifies a previously proposed control apparatus for the automatic transmission in which the revolution speed of one of the clutches which is engaged (clutched) or released during the gear shift is detected and a clutch engagement pressure is controlled in dependence upon a square of the clutch revolution speed. Thus, with an influence of the centrifugal hydraulic largely developed taken into consideration, the clutch engagement pressure is reduced by a value corresponding to the influence of the centrifugal hydraulic. Then, a more accurate engagement pressure control is made possible.
As another measure against the centrifugal hydraulic, a frictional clutching (engagement) element of the automatic transmission is well known in which a, so-called, centrifugal hydraulic cancel chamber is installed so that the centrifugal hydraulic in the working oil chamber is cancelled with the centrifugal hydraulic in the centrifugal hydraulic cancel chamber. Thus, the centrifugal hydraulic measure has been taken without carrying out the above-described control. Hereinafter, the centrifugal hydraulic cancel chamber will specifically be described. FIG. 11 shows a diagrammatical cross sectional view indicating a generally available hydraulic clutch mechanism (a frictional clutching (engagement) element) 35 of the well known automatic transmission. The hydraulic clutch mechanism 35 mainly includes a piston 40 and a hydraulic multi-plate clutch (a frictional clutching (engagement) member) 50. Hydraulic multi-plate clutch 50 is disposed so as to limit a relative revolution between an input shaft of the transmission (turbine shaft) and one element of the planetary gear mechanism (planetary carrier). A plurality of clutch discs 50b and a plurality of clutch plates 50a are disposed alternatively. It is noted that each clutch disc 50b is meshed with a spline 42 of cylinder 41 integrally rotated with turbine shaft 10. This causes an integral rotation of each clutch 50b and turbine shaft 10. In addition, a clutch operating piston 40 is fitted into cylinder 41. When the working oil is supplied into an oil chamber formed between cylinder 41 and piston 40, piston 40 is driven in a leftward direction as viewed from FIG. 11 against a biasing force of a return spring 49 and is brought in contact with clutch disc 50b. When piston 40 is driven in the way described above, piston 40 presses each clutch disc 50b so that the frictional force between each clutch plate 50a and each clutch disc 50b limits the relative revolution between turbine shaft 10 and the carrier. Thus, these members are integrally revolved.
In addition, a wall member 46 such as to cover an inside of piston 40 is disposed at an opposite side to the side at which an oil pressure (hydraulic) chamber 45 of piston 40 is formed. This wall member 46 and piston 40 forms centrifugal hydraulic cancel chamber 47. It is noted that wall member 46 is fixed by cylinder 41 and the working oil is supplied to centrifugal hydraulic cancel chamber 47 via an oil hole (not shown). Hence, during the revolution of clutch mechanism 35, especially, during a high speed revolution, due to a centrifugal force, the working oil indicates a high pressure at, especially, an outer peripheral side within oil pressure chamber 45 so that a force to try to expand a volume of oil pressure chamber 45 is developed. At this time, due to the centrifugal force, the oil within oil pressure (centrifugal hydraulic) cancel chamber 47 simultaneously indicates a high pressure and the force to try to expand the volume of centrifugal hydraulic cancel chamber 47 is developed. Hence, a force acted upon piston 40 in an axial direction is cancelled. In addition, seal rings 48a, 48b, and 48c are installed on cylinder 41, piston 40, and wall chamber 46. These seal rings 48a, 48b, 48c hermetically seal oil pressure chamber 45 and centrifugal hydraulic cancel chamber 47 and slidably supports piston 40.